"In these circumstances it would be unwise and dangerous for Britain, alone of any of the nuclear powers, to give up its independent nuclear deterrent."

He also said "it is not utterly fanciful" to "imagine states sponsoring nuclear terrorism from their soil".

MPs will vote on the plans in March after a period of debate, he said.

Conservative leader David Cameron said his party agreed with Mr Blair's position "on substance and on timing".

"It is a vital matter for our national security but it requires a long-term approach. I hope we can work together on this issue for the good of the country," he told Mr Blair.

But Liberal Democrat leader Sir Menzies Campbell said proper consideration of all relevant factors could only be made if the decision was postponed until 2014.

Security or legacy?

He added: "Why is this decision being pushed through his own Cabinet, and through Parliament, just as the prime minister is about to leave Downing Street? Is this about Britain's interests or about his legacy?"

Sir Menzies wants the number of UK warheads halved to 100 - a move he said could help kickstart multilateral disarmament.

Among Labour MPs who oppose replacing Trident former minister Michael Meacher asked: "How can this proposal really be justified in an utterly different post-Cold War environment?"

He argued that the move would restrict conventional defence spending, undermine the nuclear non-proliferation treaty and would take money away from the fights against terrorism, climate change and "long term energy insecurity".

A group of 28 Labour MPs, headed by Gordon Prentice, have written to the general secretary of the Labour Party asking for "wider and deeper consultation" among party members and affiliated organisations.

And Kate Hudson, from the anti-nuclear pressure group, CND, said she was "very very disappointed" with Mr Blair's announcement.

"He talked vaguely about reducing the number of submarines and warheads but it is not clear what that would mean," she said.

"I am sure many Labour MPs will be extremely angry because it is clear the prime minister has set out a pre-determined timetable."